The invention relates to a device for placing components on a carrier, such as a printed circuit board, comprising a frame with a vertically positioned shaft accommodated therein, which shaft is provided with a bore along its centerline, and displacement means for displacing the hollow shaft in vertical direction in the frame, which shaft can be coupled at its end facing away from the carrier to means capable of generating an underpressure or vacuum in the bore, while the end of the shaft facing towards the carrier is suitable for picking up and placing a component by means of the underpressure or vacuum.
Such a device is known from DE-39 38 088 A1. A disadvantage of such a device is the control of the impact force, i.e. the force exerted by the shaft on the component at the moment the latter comes into contact with the printed circuit board. To obtain a lead time through the device of the printed circuit board under operation which is as short as possible, the shaft is displaced in vertical direction with comparatively high velocities by the displacement means. A timely deceleration of the shaft by the displacement means is desirable in this connection for keeping the impact force as low as possible. A low mass of the moving parts is desirable here so as to be able to realize the desired high velocities and the accompanying accelerations.
However, the control of a low impact force is limited by the mass of the moving parts and the xe2x80x9creaction speedxe2x80x9d of the control in decelerating the displacement means the moment a contact with the printed circuit board is detected.
The invention has for its object to counteract the above disadvantages and to provide a device with which a very small impact force can be achieved. The device according to the invention is for this purpose characterized in that the shaft is coupled to a yoke which forms part of the displacement means, which coupling can be disengaged temporarily the moment the shaft comes into contact with a component or a carrier. A temporary disengagement at the moment of contact interrupts the vertical drive of the shaft and thus results in a very small impact force. In addition, the displacement means can also be decelerated in time owing to this temporary disengagement.
In an embodiment of the device according to the invention, the displacement means comprise a coil which is fixedly arranged in the frame, within which coil a magnet core is accommodated which is displaceable in vertical direction. The displaceable magnet core may form part of the yoke here. Since the coils of the displacement means are fixedly accommodated in the frame, a further reduction in the mass to be displaced is obtained. An additional advantage is that all connection cables for controlling the displacement means are also fixedly connected in the frame, which will considerably prolong the useful life of these cables, especially since they do not move in the frame now.
A special embodiment of a device according to the invention, in which in addition the shaft is provided with a first magnet at its end facing away from the carrier, which first magnet lays the shaft against a first abutment in the idle and in the operational position by means of a second magnet provided on the yoke, is characterized according to the invention in that the shaft is rotatably accommodated in the yoke by means of a bearing, which bearing has an axial clearance such that during operation the displacement means traverse a clearing distance from the first abutment to a second abutment in vertical direction when the shaft comes into contact with a component or a carrier. While traversing the axial clearance, the shaft is in idleness, and the control can decelerate the displacement means in time. This construction has the additional advantage that the displacement means, at the moment when they bear on the second abutment, are capable of exerting a highly controlled force on the shaft for the purpose of pressing the component into the printed circuit board.
It should be noted that U.S. Pat. No. 5,644,279 shows a placement device in which the shaft is forced against a first abutment in one direction by magnetic force. A second abutment, however, is absent here, so that the component after being placed cannot be pressed home with a defined force.
According to the invention, the second abutment is formed by a stud provided on the shaft. According to the invention, furthermore, the device may be characterized in that the displacement means are provided with reading means for reading the vertical displacement of the shaft in the frame during operation.
In a more particular embodiment of the device, the latter is characterized in that the vacuum means comprise an elongate, fixedly positioned tube which extends over a certain length from the end of the shaft facing away from the carrier into the bore. It is possible here that at least one seal is provided between the tube and the bore. As was noted above, this construction has the advantage that the vacuum means can be fixedly accommodated in the frame and need not be displaced along with the shaft in vertical direction. Here, again, only a small mass need be displaced in vertical direction through the frame.
The invention also relates to a device as described above which is in addition provided with rotation means for rotating the shaft about its centerline. According to the invention, this embodiment of the device is characterized in that the shaft is held in a housing by means of a linear ball bearing guide, said housing being rotatably accommodated in the frame. It is possible here that the housing is provided with a gearing over which a continuous transmission element, which can be driven by a motor, is provided.
The use of a linear ball bearing guide leads to a further reduction of the mass to be displaced in vertical direction.
To obtain a more accurate reading of the degree of rotation of the housing and of the shaft, according to the invention, the housing is provided with reading means for reading the rotation of the shaft during operation. In an embodiment, said reading means may comprise an encoder disc situated around the shaft.